Organic 5: Alcohols Flashcards
What is the general formula for alcohols?
ROH
CnH2n+1OH
What is the bond angle of an OH group?
The bond angle of an OH group is 105 degrees
Because the 109.5 tetrahedral angle is reduced because of two lone pairs
Lone pairs repel more than bonding pairs
Are alcohols soluble?
Shorter chain alcohols are soluble because the OH group can hydrogen bond to water molecules
Longer chain alcohols are insoluble because they have a larger non-polar hydrocarbon region, so cannot form hydrogen bonds
What are the two methods of producing ethanol?
From crude oil
Fermentation
Describe how ethanol is produced from crude oil.
Ethene is produced when crude oil fractions are cracked
Ethene can then by hydrated (in electrophilic addition reaction with water) to form ethanol
Describe how ethanol is produced by fermentation, and give an equation.
Glucose is broken down using enzymes in yeast in anaerobic respiration
C6H12O6(aq) —> 2C2H5OH(aq) + 2CO2(g)
How does temperature affect fermentation?
Enzymes can be denatured if temperatures are too high
But reaction will be too slow in low temperatures
So a compromise temperature of around 35 degrees is used
Why must the fermentation vessels be kept anaerobic?
To prevent oxidation of ethanol to ethanoic acid
How is ethanol distilled after fermentation?
Ethanol must be distilled by fractional distillation after fermentation
At boiling temperature 78 degrees Celsius
As aqueous solution produced by fermentation is impure
Compare the two methods of producing ethanol.
How can ethene be resynthesised from ethanol?
Dehydration
What is the mechanism for dehydrating ethanol?
Acid catalysed elimination
Reagent: Excess conc H2SO4 or H3PO4
Conditions: Warm, under reflux
1) Lone pair of electrons from oxygen are attracted to H+ from acid
2) Electrons in C-H bond next to the carbon attached to the OH group move to the C-C bond, and form a double C=C bond
3) Electron pair in C-O bond move to the H2O+ group, which is removed to give ethene, H+ and water
What are the advantages and disadvantages of making ethanol from crude oil?
Advantages
- Faster reaction
- Purer product
- Continuous process
Disadvantages
- High technology equipment needed
- Ethene is non-renewable resource
- High energy costs for producing high pressures
What are the advantages and disadvantages of making ethanol from fermentation?
Advantages
- Sugar is a renewable resource
- Production uses low level technology
Disadvantages
- Batch process, so is slow
- Not continous
- Ethanol is not pure, so needs purifying by fractional distillation
How is fermentation a carbon neutral process?
The carbon dioxide released is balanced by the carbon dioxide absorbed by the plant during photosynthesis
2 molecules are released in fermentation
4 molecules are released during combustion
6 molecules are absorbed during photosynthesis
Why is ethanol production from fermentation renewable?
Because it comes from plants like sugar cane, which can be replanted
What is ethanol usually used for?
Fuel e.g. in picnic stoves
What can primary alcohols be oxidised into?
Primary alcohol + [O] –> aldehyde + H2O
Primary alcohol + 2[O] –> carboxylic acid + H2O
Describe how you would carry out the oxidation of alcohols to aldehydes or ketones.
1) Using acidified potassium dichromate (K2Cr2O7 + H2SO4)
2) Orange dichromate (VI) ions are reduced to green chromium(III) ions
3) Heat and then distill off product to prevent further oxidation into carboxylic acid
Describe how you would carry out the oxidation of primary alcohols to carboxylic acids.
1) Using acidified potassium dichromate (K2Cr2O7 + H2SO4)
2) Mixture is refluxed, so vapour condenses back into the flask, so all alcohol will be oxidised into acid
Draw a diagram to show how you would distill off ethanal.
Draw a diagram to show how you would carry out a reflux reaction for oxidising an alcohol to a carboxylic acid.
Describe how you would carry out the Tollen’s test for aldehydes.
1) Add [Ag(NH3)2]+ (Tollen’s Reagent) to sample
2) Heat gently
3) Aldehydes will be oxidised into carboxylic acids
4) The silver ions in [Ag(NH3)2]+ will be reduced to silver atoms, forming a silver mirror
5) No visible change for ketones
Give an equation to show how ethanal would react with Tollen’s reagent.
CH3CHO + 2Ag+ + H2O —> CH3COOH + 2Ag + 2H+
Describe how you would carry out the Fehlings’ test for aldehydes.
1) Add Fehling’s solution containing Cu2+ ions
2) Heat gently
3) Cu2+ ions reduced to Cu+ in copper oxide, forming a red precipitate
4) No visible change for ketones
Give an equation to show how Fehling’s solution would react with ethanal.
CH3CHO + 2Cu2+ + 2H2O —> CH3COOH + CU2O + 4H+
How do you test for carboxylic acids?
1) Add sodium carbonate
2) Positive result = fizzing (carbon dioxide released)
E.g. Na2CO3 + 2CH3COOH —> 2CH3COONa + CO2 + H2O
What is a biofuel?
A fuel derived from or produced from renewable biological sources e.g. plants
What is a carbon neutral activity?
An activity that has no net carbon emissions (no emissions overall, i.e. same amount is removed as released)
